Marine screw propeller



' Jan. 16, 1934.

E. v. TELFER 1,943,934 I MARINE SCREW PROPELLER Filed Nov. 5, 1931 Q MW 6W m Ji/z w,

Patented Jan. 16, 1934 snares PATENT OFFICE MARINE scaaw PROPELLER Edmund Victor Telfer, Newcastle-upon-Tyne, England 4 Claims.

This invention relates to improvements in marine screw propellers and is concerned with the avoidance of erosion in such propellers. The invention is principally applicable to the 5 propellers of high powered and/or high speed vessels, although it may also be applicable to the propellers of low or medium powered vessels having blades made of cast iron, for example. It willalso be understood that the invention may be applied to vessels having single or multiple propellers.

Erosion of propellers is due to a Water hammer effect produced by the eduction of socalled vacuous cavities in a region of low absolute fluid pressure and the collapse of such cavities in a region of sufliciently high pressure. Under some Working conditions, for example, the transition from suction to pressure over the propeller driving face takes place very rapidly and within a short linear extent of the blade edge. Vacuous cavities formed in the suction zone are collapsed in the pressure zone, and the resultant water hammer effect erodes the driving face.

On the back face of the blade, the vacuous cavities formed in the region of the peak of the suction, collapse when the suction is sufficiently reduced, and hence erosion may be set up in the collapse region.

It is found in some cases that erosion occurs principally on the driving face of the propeller blade.

The object of the present invention is to eliminate or minimize erosion on one or both faces of the blade, as found to be necessary, by preventing the formation of the vacuous cavities. A further object is to provide means for eliminating or minimizing erosion and still leave the design of the propeller free to be determined 40 from considerations of maximum propulsive efficiency. Attendant advantages are derived, since the shock transmitted to the hull of the ship is avoided owing to the reduced propeller cavitation. Thus the comfort of the ship is increased owing to the reduction of vibration; and higher speeds and higher powers may be developed with safety. The shaft and hull fatigue are moreover reduced whereby the safety and strength of the vessel may be increased. In addition thinner blades may be used in view of the reduction of stresses, while the reduction in cavitation may enable the area of the blades to be reduced with corresponding increase in 55 efficiency and reduction in weight and manufacturing costs.

According to this invention the object aforesaid is achieved by roughening a marginal portion at or near the leading edge of the driving face so as to brake the water flow and prevent the higher initial velocity from producing local suction, the area of roughening covering an area which would otherwise be a suction area, and its extent may be determined by calculation.

The roughening may take the form of grooves applied to the blade by a suitable hand tool having appropriate means for determining the depth of the grooves. The latter may vary in depth and may be graduated in accordance With requirements. Thus the grooves may be fine at the leading edge part, then gradually increase in depth, and finally gradually fade into the smooth face of the blade. The roughening need not commence at the leading edge. For example when used in combination With blades having a tilted leading edge the roughening may commence at some distance from that edge. Alternatively, the roughening may take the form of knuriings, scorings, or the like. The roughening may also take other forms adapted to produce a braking of the water stream and pre- 8 vent high initial velocity on the driving face and the corresponding formation of local suction. For example the area to be roughened may be formed with closely arranged centre point dents or like indentations. Or projections may be formed or produced on the blade. Also the blade may be formed with a suitably roughened marginal edge portion during the casting operation for its production. The roughening whilst eliminating or minimizing erosion enables the 9. propeller blades to be designed With regard to maximum propulsive efficiency since the roughening places no restriction on the design of the propeller blades. The roughening does not detract froin the efficiency of the propeller in other directions, but avoidance of suction on the driving face may result in a gain in thrust in View of the increased pressure on the driving face.

The roughening may also be applied to the back face of the blade at or near the leadingl edge. In this Way the peak of the suction existing in the region of this edge as previously referred to may be reduced and the pressure gradi ent from the leading to the trailing edge lowered. Thus the suction over the back of the blade is more uniformly distributed. Therefore, there is only a gradual transition from the suction at the leading edge to the absolute pressure at the following or trailing edge. A degree of roughness is adopted sufficient to reduce the suction to such an extent that vacuous cavities will not be formed, or will not be violently collapsed after formation. In addition the uniformity of suction distribution over the blade enables the blade to be of reduced area.

In order to assist the understanding of the invention, reference will be made to the accompanying drawing illustrating expedients for achieving the object of the invention, in which drawing:

Figure 1 is an end elevation of a screw propeller boss and blade, the latter having roughenings applied to its driving face in accordance with these improvements.

Figure 2 is a section, to a larger scale, on the line II-II, Figure 1, and

Figure 3 is a view similar to Figure 2 showing the application of roughenings to the back of a blade.

Referring to Figures 1 and 2 the blade a is formed or provided with roughenings b on its driving face adjacent the leading edge 0 as hereinbefore described. As shown the area and location of the roughening is suitable for a lightly loaded propeller but naturally it may be modified or extended in accordance with different requirements. As seen in Figure 2, the roughening commences at or near the leading edge 0 in the form of fine grooves or the like 0 which gradually increase in depth at c and then become finer again at 0 until they gradually fade into the smooth driving face of the blade.

Figure 3 shows a portion of a section at an outer radius of the blade of a more heavily loaded propeller than that shown in Figures 1 and 2 and illustrates the initial roughening d on the back of the blade.

I claim:

1. Screw propeller comprising blades having their faces roughened on marginal portions adjacent their leading edges, said roughenings be ing fine at the leading edge part, then gradually increasing in coarseness and finally gradually fading into the smooth surface of the blade and operative for establishing positive pressure areas over predetermined blade areas by resisting the flow of water over the blade.

2. Marine propeller blade having a leading portion of its driving face formed with a roughened area displaced from the leading edge, and stopping short of the tip, the roughening being produced by numerous depressions in the driving face and adapted for resisting flow of liquid across such face from the leading edge towards the trailing edge.

3. Marine propeller blade having a leading portion ;of its driving face provided with a roughened area displaced from the leading edge, the said roughened area being produced by numerous grooves in the direction from hub to tip but stopping short of the tip.

4:. Marine propeller blade having the leading edge portion of its driving face formed with a roughened area displaced from the leading edge and formed by numerous grooves disposed generally in the direction from root to tip and adapted for resisting the flow of liquid across the face from the leading edge towards the trailing edge, said roughened area terminating short of the tip and being of decreasing width from the root outwards.

EDMUND VICTOR TELFER. 

